Climate change

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For current and future climatological effects of human influences, see global warming. For
the study of past climate change, see paleoclimatology. For temperatures on the longest time
scales, see geologic temperature record.
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Climate change is a significant and lasting change in the statistical distribution of weather
patterns over periods ranging from decades to millions of years. It may be a change in
average weather conditions or the distribution of events around that average (e.g., more or
fewer extreme weather events). Climate change may be limited to a specific region or may
occur across the whole Earth.

Contents
[hide]
  1 Terminology
 2 Causes
o 2.1 Internal forcing mechanisms
 2.1.1 Ocean variability
o 2.2 External forcing mechanisms
 2.2.1 Orbital variations
 2.2.2 Solar output
 2.2.3 Volcanism
 2.2.4 Plate tectonics
 2.2.5 Human influences
 3 Physical evidence for and examples of climatic change
o 3.1 Temperature measurements and proxies
o 3.2 Historical and archaeological evidence
o 3.3 Glaciers
o 3.4 Arctic sea ice loss
o 3.5 Vegetation
o 3.6 Pollen analysis
o 3.7 Precipitation
o 3.8 Dendroclimatology
o 3.9 Ice cores
o 3.10 Animals
o 3.11 Sea level change
 4 See also
 5 Notes
 6 References
 7 Further reading

 8 External links

Terminology
The most general definition of climate change is a change in the statistical properties of the
climate system when considered over long periods of time, regardless of cause.[1]
Accordingly, fluctuations over periods shorter than a few decades, such as El Niño, do not
represent climate change.

The term sometimes is used to refer specifically to climate change caused by human activity,
as opposed to changes in climate that may have resulted as part of Earth's natural processes. [2]
In this latter sense, used especially in the context of environmental policy, the term climate
change today is synonymous with anthropogenic global warming. Within scientific journals,
however, global warming refers to surface temperature increases, while climate change
includes global warming and everything else that increasing greenhouse gas amounts will
affect.[3]

Causes
Climate changes in response to changes in the global energy balance. On the broadest scale,
the rate at which energy is received from the sun and the rate at which it is lost to space
determine the equilibrium temperature and climate of Earth. This energy is then distributed
around the globe by winds, ocean currents, and other mechanisms to affect the climates of
different regions.

Factors that can shape climate are called climate forcings or "forcing mechanisms".[4] These
include such processes as variations in solar radiation, deviations in the Earth's orbit,
mountain-building and continental drift, and changes in greenhouse gas concentrations. There
are a variety of climate change feedbacks that can either amplify or diminish the initial
forcing. Some parts of the climate system, such as the oceans and ice caps, respond slowly in
reaction to climate forcings, while others respond more quickly.

Forcing mechanisms can be either "internal" or "external". Internal forcing mechanisms are
natural processes within the climate system itself (e.g., the meridional overturning
circulation). External forcing mechanisms can be either natural (e.g., changes in solar output)
or anthropogenic (e.g., increased emissions of greenhouse gases).

Whether the initial forcing mechanism is internal or external, the response of the climate
system might be fast (e.g., a sudden cooling due to airborne volcanic ash reflecting sunlight),
slow (e.g. thermal expansion of warming ocean water), or a combination (e.g., sudden loss of
albedo in the arctic ocean as sea ice melts, followed by more gradual thermal expansion of
the water). Therefore, the climate system can respond abruptly, but the full response to
forcing mechanisms might not be fully developed for centuries or even longer.

Internal forcing mechanisms

Natural changes in the components of earth's climate system and their interactions are the
cause of internal climate variability, or "internal forcings." Scientists generally define the five
components of earth's climate system to include Atmosphere, hydrosphere, cryosphere,
lithosphere (restricted to the surface soils, rocks, and sediments), and biosphere.[5][citation needed]

Ocean variability

Pacific Decadal Oscillation 1925 to 2010

Main article: Thermohaline circulation

The ocean is a fundamental part of the climate system, some changes in it occurring at longer
timescales than in the atmosphere, massing hundreds of times more and having very high
thermal inertia (such as the ocean depths still lagging today in temperature adjustment from
the Little Ice Age).[6]

Short-term fluctuations (years to a few decades) such as the El Niño-Southern Oscillation, the
Pacific decadal oscillation, the North Atlantic oscillation, and the Arctic oscillation, represent
climate variability rather than climate change. On longer time scales, alterations to ocean
processes such as thermohaline circulation play a key role in redistributing heat by carrying
out a very slow and extremely deep movement of water, and the long-term redistribution of
heat in the world's oceans.

A schematic of modern thermohaline circulation. Tens of millions of years ago, continental

plate movement formed a land-free gap around Antarctica, allowing formation of the ACC
which keeps warm waters away from Antarctica.

External forcing mechanisms

Increase in Atmospheric CO2 Levels

Milankovitch cycles from 800,000 years ago in the past to 800,000 years in the future.
Variations in CO2, temperature and dust from the Vostok ice core over the last 450,000 years

Orbital variations

Main article: Milankovitch cycles

Slight variations in Earth's orbit lead to changes in the seasonal distribution of sunlight
reaching the Earth's surface and how it is distributed across the globe. There is very little
change to the area-averaged annually averaged sunshine; but there can be strong changes in
the geographical and seasonal distribution. The three types of orbital variations are variations
in Earth's eccentricity, changes in the tilt angle of Earth's axis of rotation, and precession of
Earth's axis. Combined together, these produce Milankovitch cycles which have a large
impact on climate and are notable for their correlation to glacial and interglacial periods,[7]
their correlation with the advance and retreat of the Sahara,[7] and for their appearance in the
stratigraphic record.[8]

The IPCC notes that Milankovitch cycles drove the ice age cycles; CO2 followed temperature
change "with a lag of some hundreds of years"; and that as a feedback amplified temperature
change.[9] The depths of the ocean have a lag time in changing temperature (thermal inertia on
such scale). Upon seawater temperature change, the solubility of CO2 in the oceans changed,
as well as other factors impacting air-sea CO2 exchange.[10]

Solar output

Main article: Solar variation

Variations in solar activity during the last several centuries based on observations of sunspots
and beryllium isotopes. The period of extraordinarily few sunspots in the late 17th century
was the Maunder Minimum.

The sun is the predominant source for energy input to the Earth. Both long- and short-term
variations in solar intensity are known to affect global climate.

Three to four billion years ago the sun emitted only 70% as much power as it does today. If
the atmospheric composition had been the same as today, liquid water should not have
existed on Earth. However, there is evidence for the presence of water on the early Earth, in
the Hadean[11][12] and Archean[13][11] eons, leading to what is known as the faint young Sun
paradox.[14] Hypothesized solutions to this paradox include a vastly different atmosphere, with
much higher concentrations of greenhouse gases than currently exist.[15] Over the following
approximately 4 billion years, the energy output of the sun increased and atmospheric
composition changed. The Great Oxygenation Event -oxygenation of the atmosphere- around
2.4 billion years ago was the most notable alteration. Over the next five billion years the sun's
ultimate death as it becomes a red giant and then a white dwarf will have large effects on
climate, with the red giant phase possibly ending any life on Earth that survives until that
time.

Solar output also varies on shorter time scales, including the 11-year solar cycle[16] and
longer-term modulations.[17] Solar intensity variations are considered to have been influential
in triggering the Little Ice Age,[18] and some of the warming observed from 1900 to 1950. The
cyclical nature of the sun's energy output is not yet fully understood; it differs from the very
slow change that is happening within the sun as it ages and evolves. Research indicates that
solar variability has had effects including the Maunder Minimum from 1645 to 1715 A.D.,
part of the Little Ice Age from 1550 to 1850 A.D. which was marked by relative cooling and
greater glacier extent than the centuries before and afterward.[19][20] Some studies point toward
solar radiation increases from cyclical sunspot activity affecting global warming, and climate
may be influenced by the sum of all effects (solar variation, anthropogenic radiative forcings,
etc.).[21][22]

Interestingly, a 2010 study[23] suggests, “that the effects of solar variability on temperature
throughout the atmosphere may be contrary to current expectations.”

In an Aug 2011 Press Release,[24] CERN announced the publication in the Nature journal the
initial results from its CLOUD experiment. The results indicate that ionisation from cosmic
rays significantly enhances aerosol formation in the presence of sulphuric acid and water, but
in the lower atmosphere where ammonia is also required, this is insufficient to account for
aerosol formation and additional trace vapours must be involved. The next step is to find
more about these trace vapours, including whether they are of natural or human origin.

Further information: Cosmic ray#Role_in_climate_change

Volcanism

In atmospheric temperature from 1979 to 2010, determined by MSU NASA satellites, effects
appear from aerosols released by major volcanic eruptions (El Chichón and Pinatubo).
El Niño is a separate event, from ocean variability.

Volcanic eruptions release gases and particulates into the atmosphere. Eruptions large enough
to affect climate occur on average several times per century, and cause cooling (by partially
blocking the transmission of solar radiation to the Earth's surface) for a period of a few years.
The eruption of Mount Pinatubo in 1991, the second largest terrestrial eruption of the 20th
century[25] (after the 1912 eruption of Novarupta[26]) affected the climate substantially. Global
temperatures decreased by about 0.5 °C (0.9 °F). The eruption of Mount Tambora in 1815
caused the Year Without a Summer.[27] Much larger eruptions, known as large igneous
provinces, occur only a few times every hundred million years, but may cause global
warming and mass extinctions.[28]
Volcanoes are also part of the extended carbon cycle. Over very long (geological) time
periods, they release carbon dioxide from the Earth's crust and mantle, counteracting the
uptake by sedimentary rocks and other geological carbon dioxide sinks. The US Geological
Survey estimates are that volcanic emissions are at a much lower level than than the effects of
current human activities, which generate 100-300 times the amount of carbon dioxide emitted
by volcanoes.[29] A review of published studies indicates that annual volcanic emissions of
carbon dioxide, including amounts released from mid-ocean ridges, volcanic arcs, and hot
spot volcanoes, are only the equivalent of 3 to 5 days of human caused output. The annual
amount put out by human activities may be greater than the amount released by
supererruptions, the most recent of which was the Toba eruption in Indonesia 74,000 years
ago.[30]

Although volcanoes are technically part of the lithosphere, which itself is part of the climate
system, the IPCC explicitly defines volcanism as an external forcing agent.[31]

Plate tectonics

Over the course of millions of years, the motion of tectonic plates reconfigures global land
and ocean areas and generates topography. This can affect both global and local patterns of
climate and atmosphere-ocean circulation.[32]

The position of the continents determines the geometry of the oceans and therefore influences
patterns of ocean circulation. The locations of the seas are important in controlling the
transfer of heat and moisture across the globe, and therefore, in determining global climate. A
recent example of tectonic control on ocean circulation is the formation of the Isthmus of
Panama about 5 million years ago, which shut off direct mixing between the Atlantic and
Pacific Oceans. This strongly affected the ocean dynamics of what is now the Gulf Stream
and may have led to Northern Hemisphere ice cover.[33][34] During the Carboniferous period,
about 300 to 360 million years ago, plate tectonics may have triggered large-scale storage of
carbon and increased glaciation.[35] Geologic evidence points to a "megamonsoonal"
circulation pattern during the time of the supercontinent Pangaea, and climate modeling
suggests that the existence of the supercontinent was conducive to the establishment of
monsoons.[36]

The size of continents is also important. Because of the stabilizing effect of the oceans on
temperature, yearly temperature variations are generally lower in coastal areas than they are
inland. A larger supercontinent will therefore have more area in which climate is strongly
seasonal than will several smaller continents or islands.

Human influences

Main article: Global warming

Main article: Climate change mitigation

In the context of climate variation, anthropogenic factors are human activities which affect
the climate. The scientific consensus on climate change is "that climate is changing and that
these changes are in large part caused by human activities,"[37] and it "is largely
irreversible."[38]
“Science has made enormous inroads in understanding climate change and its causes, and is
beginning to help develop a strong understanding of current and potential impacts that will
affect people today and in coming decades. This understanding is crucial because it allows
decision makers to place climate change in the context of other large challenges facing the
nation and the world. There are still some uncertainties, and there always will be in
understanding a complex system like Earth’s climate. Nevertheless, there is a strong, credible
body of evidence, based on multiple lines of research, documenting that climate is changing
and that these changes are in large part caused by human activities. While much remains to
be learned, the core phenomenon, scientific questions, and hypotheses have been examined
thoroughly and have stood firm in the face of serious scientific debate and careful evaluation
of alternative explanations.”

— United States National Research Council, Advancing the Science of Climate Change

Of most concern in these anthropogenic factors is the increase in CO2 levels due to emissions
from fossil fuel combustion, followed by aerosols (particulate matter in the atmosphere) and
cement manufacture. Other factors, including land use, ozone depletion, animal agriculture[39]
and deforestation, are also of concern in the roles they play - both separately and in
conjunction with other factors - in affecting climate, microclimate, and measures of climate
variables.

Physical evidence for and examples of climatic change

Comparisons between Asian Monsoons from 200 A.D. to 2000 A.D. (staying in the
background on other plots), Northern Hemisphere temperature, Alpine glacier extent
(vertically inverted as marked), and human history as noted by the U.S. NSF.

Arctic temperature anomalies over a 100 year period as estimated by NASA. Typical high
monthly variance can be seen, while longer-term averages highlight trends.
Evidence for climatic change is taken from a variety of sources that can be used to
reconstruct past climates. Reasonably complete global records of surface temperature are
available beginning from the mid-late 19th century. For earlier periods, most of the evidence
is indirect—climatic changes are inferred from changes in proxies, indicators that reflect
climate, such as vegetation, ice cores,[40] dendrochronology, sea level change, and glacial
geology.

Temperature measurements and proxies

The instrumental temperature record from surface stations was supplemented by radiosonde
balloons, extensive atmospheric monitoring by the mid-20th century, and, from the 1970s on,
with global satellite data as well. The 18O/16O ratio in calcite and ice core samples used to
deduce ocean temperature in the distant past is an example of a temperature proxy method, as
are other climate metrics noted in subsequent categories.

Historical and archaeological evidence

Main article: Historical impacts of climate change

Climate change in the recent past may be detected by corresponding changes in settlement
and agricultural patterns.[41] Archaeological evidence, oral history and historical documents
can offer insights into past changes in the climate. Climate change effects have been linked to
the collapse of various civilizations.[41]

Decline in thickness of glaciers worldwide over the past half-century

Glaciers

Glaciers are considered among the most sensitive indicators of climate change.[42] Their size
is determined by a mass balance between snow input and melt output. As temperatures warm,
glaciers retreat unless snow precipitation increases to make up for the additional melt; the
converse is also true.

Glaciers grow and shrink due both to natural variability and external forcings. Variability in
temperature, precipitation, and englacial and subglacial hydrology can strongly determine the
evolution of a glacier in a particular season. Therefore, one must average over a decadal or
longer time-scale and/or over a many individual glaciers to smooth out the local short-term
variability and obtain a glacier history that is related to climate.
A world glacier inventory has been compiled since the 1970s, initially based mainly on aerial
photographs and maps but now relying more on satellites. This compilation tracks more than
100,000 glaciers covering a total area of approximately 240,000 km2, and preliminary
estimates indicate that the remaining ice cover is around 445,000 km2. The World Glacier
Monitoring Service collects data annually on glacier retreat and glacier mass balance From
this data, glaciers worldwide have been found to be shrinking significantly, with strong
glacier retreats in the 1940s, stable or growing conditions during the 1920s and 1970s, and
again retreating from the mid 1980s to present.[43]

The most significant climate processes since the middle to late Pliocene (approximately 3
million years ago) are the glacial and interglacial cycles. The present interglacial period (the
Holocene) has lasted about 11,700 years.[44] Shaped by orbital variations, responses such as
the rise and fall of continental ice sheets and significant sea-level changes helped create the
climate. Other changes, including Heinrich events, Dansgaard–Oeschger events and the
Younger Dryas, however, illustrate how glacial variations may also influence climate without
the orbital forcing.

Glaciers leave behind moraines that contain a wealth of material—including organic matter,
quartz, and potassium that may be dated—recording the periods in which a glacier advanced
and retreated. Similarly, by tephrochronological techniques, the lack of glacier cover can be
identified by the presence of soil or volcanic tephra horizons whose date of deposit may also
be ascertained.

This time series, based on satellite data, shows the annual Arctic sea ice minimum since
1979. The September 2010 extent was the third lowest in the satellite record.

Arctic sea ice loss

Main articles: Polar ice packs and Climate change in the Arctic

The decline in Arctic sea ice, both in extent and thickness, over the last several decades is
further evidence for rapid climate change.[45] Sea ice is frozen seawater that floats on the
ocean surface. It covers millions of square miles in the polar regions, varying with the
seasons. In the Arctic, some sea ice remains year after year, whereas almost all Southern
Ocean or Antarctic sea ice melts away and reforms annually. Satellite observations show that
Arctic sea ice is now declining at a rate of 11.5 percent per decade, relative to the 1979 to
2000 average.[46]
This video summarizes how climate change, associated with increased carbon dioxide levels,
has affected plant growth.

Vegetation

A change in the type, distribution and coverage of vegetation may occur given a change in
the climate. Some changes in climate may result in increased precipitation and warmth,
resulting in improved plant growth and the subsequent sequestration of airborne CO2. Larger,
faster or more radical changes, however, may result in vegetation stress, rapid plant loss and
desertification in certain circumstances.[47][48] An example of this occurred during the
Carboniferous Rainforest Collapse (CRC), an extinction event 300 million years ago. At this
time vast rainforests covered the equatorial region of Europe and America. Climate change
devastated these tropical rainforests, abruptly fragmenting the habitat into isolated 'islands'
and causing the extinction of many plant and animal species.[47]

Satellite data available in recent decades indicates that global terrestrial net primary
production increased by 6% from 1982 to 1999, with the largest portion of that increase in
tropical ecosystems, then decreased by 1% from 2000 to 2009.[49][50]

Pollen analysis

Palynology is the study of contemporary and fossil palynomorphs, including pollen.

Palynology is used to infer the geographical distribution of plant species, which vary under
different climate conditions. Different groups of plants have pollen with distinctive shapes
and surface textures, and since the outer surface of pollen is composed of a very resilient
material, they resist decay. Changes in the type of pollen found in different layers of sediment
in lakes, bogs, or river deltas indicate changes in plant communities. These changes are often
a sign of a changing climate.[51][52] As an example, palynological studies have been used to
track changing vegetation patterns throughout the Quaternary glaciations[53] and especially
since the last glacial maximum.[54]
Top: Arid ice age climate
Middle: Atlantic Period, warm and wet
Bottom: Potential vegetation in climate now if not for human effects like agriculture. [55]

Precipitation

Past precipitation can be estimated in the modern era with the global network of precipitation
gauges. Surface coverage over oceans and remote areas is relatively sparse, but, reducing
reliance on interpolation, satellite data has been available since the 1970s.[56] Quantification
of climatological variation of precipitation in prior centuries and epochs is less complete but
approximated using proxies such as marine sediments, ice cores, cave stalagmites, and tree
rings.[57]

Climatological temperatures substantially affect precipitation. For instance, during the Last
Glacial Maximum of 18,000 years ago, thermal-driven evaporation from the oceans onto
continental landmasses was low, causing large areas of extreme desert, including polar
deserts (cold but with low rates of precipitation).[55] In contrast, the world's climate was wetter
than today near the start of the warm Atlantic Period of 8000 years ago.[55]

Estimated global land precipitation increased by approximately 2% over the course of the
20th century, though the calculated trend varies if different time endpoints are chosen,
complicated by ENSO and other oscillations, including greater global land precipitation in
the 1950s and 1970s than the later 1980s and 1990s despite the positive trend over the
century overall.[56][58][59] Similar slight overall increase in global river runoff and in average
soil moisture has been perceived.[58]

Dendroclimatology
Dendroclimatology is the analysis of tree ring growth patterns to determine past climate
variations.[60] Wide and thick rings indicate a fertile, well-watered growing period, whilst
thin, narrow rings indicate a time of lower rainfall and less-than-ideal growing conditions.

Ice cores

Analysis of ice in a core drilled from a ice sheet such as the Antarctic ice sheet, can be used
to show a link between temperature and global sea level variations. The air trapped in bubbles
in the ice can also reveal the CO2 variations of the atmosphere from the distant past, well
before modern environmental influences. The study of these ice cores has been a significant
indicator of the changes in CO2 over many millennia, and continues to provide valuable
information about the differences between ancient and modern atmospheric conditions.

Animals

Remains of beetles are common in freshwater and land sediments. Different species of
beetles tend to be found under different climatic conditions. Given the extensive lineage of
beetles whose genetic makeup has not altered significantly over the millennia, knowledge of
the present climatic range of the different species, and the age of the sediments in which
remains are found, past climatic conditions may be inferred.[61]

Variation in Pacific salmon catch over the 20th century and correlation with a climate-related
Atmospheric Circulation Index (ACI) as estimated by the U.N. FAO.

Similarly, the historical abundance of various fish species has been found to have a
substantial relationships with observed climatic conditions .[62] Changes in the primary
productivity of autotrophs in the oceans can affect marine food webs.[63]

Sea level change

Global sea level change for much of the last century has generally been estimated using tide
gauge measurements collated over long periods of time to give a long-term average. More
recently, altimeter measurements — in combination with accurately determined satellite
orbits — have provided an improved measurement of global sea level change.[64] To measure
sea levels prior to instrumental measurements, scientists have dated coral reefs that grow near
the surface of the ocean, coastal sediments, marine terraces, ooids in limestones, and
nearshore archaeological remains. The predominant dating methods used are uranium series
and radiocarbon, with cosmogenic radionuclides being sometimes used to date terraces that
have experienced relative sea level fall.